High temperature oxidation protection for composites

ABSTRACT

The present disclosure provides a method for coating a composite structure, comprising forming a first slurry by combining a first pre-slurry composition with a first carrier fluid, applying the first slurry on a surface of the composite structure, and heating the composite structure to a temperature sufficient to form a base layer on the composite structure. The first pre-slurry composition may comprise a first phosphate glass composition and a low coefficient of thermal expansion material, wherein the low coefficient of thermal expansion material is a material with a coefficient of thermal expansion of less than 10×10−6° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to and thebenefit of, U.S. Ser. No. 15/169,257 filed May 31, 2016 and entitled“HIGH TEMPERATURE OXIDATION PROTECTION FOR COMPOSITES,” which is herebyincorporated by reference in its entirety for all purposes FIELD

The present disclosure relates generally to carbon-carbon compositesand, more specifically, to oxidation protection systems forcarbon-carbon composite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typicallydesigned to minimize loss of carbon material due to oxidation atoperating conditions, which include temperatures as high as 900° C.(1652° F.). Phosphate-based oxidation protection systems may reduceinfiltration of oxygen and oxidation catalysts into the compositestructure. However, despite the use of such oxidation protectionsystems, significant oxidation of the carbon-carbon composites may stilloccur during operation of components such as, for example, aircraftbraking systems.

SUMMARY

A method for coating a composite structure is provided comprisingforming a first slurry by combining a first pre-slurry composition witha first carrier fluid, applying the first slurry on a surface of thecomposite structure, and heating the composite structure to atemperature sufficient to form a base layer on the composite structure.The first pre-slurry composition may comprise a first phosphate glasscomposition, and a low coefficient of thermal expansion material,wherein the low coefficient of thermal expansion material is a materialwith a coefficient of thermal expansion of less than 10×10⁻⁶° C.⁻¹.

In various embodiments, the low coefficient of thermal expansionmaterial may comprise at least one of beta-spodumene, beta-eucryptite,cordierite, faujasite, CaZr₄P₆O₂₄, SrZr₄P6O₂₄, NaZr₂P₃O₁₂, MoZr₂P₂O₁₂,WZr₂P₂O₁₂, Al₂Mo₃O₁₂, Al₂W₃O₁₂, niobium pentoxide, aluminum titanate,Zr₂P₂O₉, beryl, silicon dioxide glass, SiO₂—TiO₂ glass, Cu₂O—Al₂O₃—SiO₂glasses, a material comprising, by weight, 57.2% SiO₂, 25.3% Al₂O₃, 6.5%P₂O₅, 3.4% Li₂O, 2.3% TiO₂, 1.8% ZrO₂, 1.4% ZnO, 1.0% MgO, 0.5% As₂O₃,0.4% K₂O, and 0.2% Na₂O, or lead magnesium niobate.

In various embodiments, the method may further comprise forming a secondslurry by combining a second pre-slurry composition with a secondcarrier fluid, wherein the second pre-slurry composition comprises asecond phosphate glass composition, applying the second slurry to thebase layer, and/or heating the composite structure to a secondtemperature sufficient to form a sealing layer on the base layer. Invarious embodiments, the method may further comprise applying at leastone of a pretreating composition or a barrier coating to the compositestructure prior to applying the first slurry to the composite structure.

In various embodiments, the method may further comprise applying apretreating composition, wherein the applying may comprise applying afirst pretreating composition to an outer surface of the compositestructure, the first pretreating composition comprising aluminum oxideand water, heating the pretreating composition, and/or applying a secondpretreating composition comprising at least one of a phosphoric acid oran acid phosphate salt, and an aluminum salt on the first pretreatingcomposition, wherein the composite structure is porous and the secondpretreating composition penetrates at least a pore of the compositestructure. The barrier coating may comprise at least one of a carbide, anitride, a boron nitride, a silicon carbide, a titanium carbide, a boroncarbide, a silicon oxycarbide, a molybdenum disulfide, a tungstendisulfide, or a silicon nitride. In various embodiments, the method mayfurther comprise applying a barrier coating by at least one of reactingthe composite structure with molten silicon, spraying, chemical vapordeposition (CVD), molten application, or brushing.

In various embodiments, the first pre-slurry composition of the baselayer may comprise between about 15 weight percent and about 30 weightpercent of boron nitride. In various embodiments, the first phosphateglass composition and/or the second phosphate glass composition may berepresented by the formulaa(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z):

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, andmixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium,manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead,zirconium, lanthanum, cerium, praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium,calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y₁+y₂+z)=1; and

x<(y₁+y₂).

In various embodiments, the first slurry may comprise a refractorycompound such as a nitride, a boron nitride, a silicon carbide, atitanium carbide, a boron carbide, a silicon oxycarbide, siliconnitride, molybdenum disulfide, or tungsten disulfide. In variousembodiments, at least one of the first slurry or the second slurrycomprises at least one of a surfactant, a flow modifier, a polymer,ammonium hydroxide, ammonium dihydrogen phosphate, acid aluminumphosphate, nanoplatelets, or graphene nanoplatelets.

In various embodiments, a slurry for coating a composite structure maycomprise a carrier fluid and a first pre-slurry composition. The firstpre-slurry composition may comprise a first phosphate glass compositionand a low coefficient of thermal expansion material, wherein the lowcoefficient of thermal expansion material is a material with acoefficient of thermal expansion of less than 10×10⁻⁶° C.⁻¹. In variousembodiments, the low coefficient of thermal expansion material maycomprise at least one of beta-spodumene, beta-eucryptite, cordierite,faujasite, CaZr₄P₆O₂₄, SrZr₄P6O₂₄, NaZr₂P₃O₁₂, MoZr₂P₂O₁₂, WZr₂P₂O₁₂,Al₂Mo₃O₁₂, Al₂W₃O₁₂, niobium pentoxide, aluminum titanate, Zr₂P₂O₉,beryl, silicon dioxide glass, SiO₂—TiO₂ glass, Cu₂O—Al₂O₃—SiO₂ glasses,a material comprising, by weight, 57.2% SiO₂, 25.3% Al₂O₃, 6.5% P₂O₅,3.4% Li₂O, 2.3% TiO₂, 1.8% ZrO₂, 1.4% ZnO, 1.0% MgO, 0.5% As₂O₃, 0.4%K₂O, and 0.2% Na₂O, or lead magnesium niobate.

In various embodiments, the first pre-slurry composition comprisesbetween about 15 weight percent and 30 weight percent boron nitride. Invarious embodiments, the first phosphate glass composition may berepresented by the formula a(A′2O)x(P2O5)y1b(GfO)y2c(A″O)z:

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, andmixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium,manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead,zirconium, lanthanum, cerium, praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium,calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y₁+y₂+z)=1; and

x<(y₁+y₂).

In various embodiments, the slurry may further comprise a refractorycompound such as a nitride, a boron nitride, a silicon carbide, atitanium carbide, a boron carbide, a silicon oxycarbide, siliconnitride, molybdenum disulfide, or tungsten disulfide. In variousembodiments, the slurry may further comprise at least one of asurfactant, a flow modifier, a polymer, ammonium hydroxide, ammoniumdihydrogen phosphate, acid aluminum phosphate, nanoplatelets, orgraphene nanoplatelets.

In various embodiments, an article is provided comprising acarbon-carbon composite structure, an oxidation protection compositionincluding a base layer disposed on an outer surface of the carbon-carboncomposite structure, wherein the base layer comprises a first pre-slurrycomposition having a low coefficient of thermal expansion material,wherein the low coefficient of thermal expansion material is a materialwith a coefficient of thermal expansion of less than 10×10⁻⁶° C.⁻¹. Invarious embodiments, the material with a low coefficient of thermalexpansion material may comprise at least one of beta-spodumene,beta-eucryptite, cordierite, faujasite, CaZr₄P₆O₂₄, SrZr₄P₆O₂₄,NaZr₂P₃O₁₂, MoZr₂P₂O₁₂, WZr₂P₂O₁₂, Al₂Mo₃O₁₂, Al₂W₃O₁₂, niobiumpentoxide, aluminum titanate, Zr₂P₂O₉, beryl, silicon dioxide glass,SiO₂—TiO₂ glass, Cu₂O—Al₂O₃—SiO₂ glasses, a material comprising, byweight, 57.2% SiO₂, 25.3% Al₂O₃, 6.5% P₂O₅, 3.4% Li₂O, 2.3% TiO₂, 1.8%ZrO₂, 1.4% ZnO, 1.0% MgO, 0.5% As₂O₃, 0.4% K₂O, and 0.2% Na₂O, or leadmagnesium niobate.

In various embodiments, the article may comprise a sealing layer on anouter surface of the base layer, wherein the sealing layer comprises asecond pre-slurry composition. The second pre-slurry composition maycomprise a second phosphate glass composition and/or acid aluminumphosphate.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed outand distinctly claimed in the concluding portion of the specification. Amore complete understanding of the present disclosure, however, may bestbe obtained by referring to the detailed description and claims whenconsidered in connection with the drawing figures, wherein like numeralsdenote like elements.

FIG. 1A illustrates a cross sectional view of an aircraft wheel brakingassembly, in accordance with various embodiments;

FIG. 1B illustrates a partial side view of an aircraft wheel brakingassembly, in accordance with various embodiments; and

FIGS. 2A, 2B, and 2C illustrate a method for coating a compositestructure in accordance with various embodiments.

DETAILED DESCRIPTION

The detailed description of embodiments herein makes reference to theaccompanying drawings, which show embodiments by way of illustration.While these embodiments are described in sufficient detail to enablethose skilled in the art to practice the disclosure, it should beunderstood that other embodiments may be realized and that logical andmechanical changes may be made without departing from the spirit andscope of the disclosure. Thus, the detailed description herein ispresented for purposes of illustration only and not for limitation. Forexample, any reference to singular includes plural embodiments, and anyreference to more than one component or step may include a singularembodiment or step. Also, any reference to attached, fixed, connected orthe like may include permanent, removable, temporary, partial, fulland/or any other possible attachment option.

With initial reference to FIGS. 1A and 1B, aircraft wheel brakingassembly 10 such as may be found on an aircraft, in accordance withvarious embodiments is illustrated. Aircraft wheel braking assembly may,for example, comprise a bogie axle 12, a wheel 14 including a hub 16 anda wheel well 18, a web 20, a torque take-out assembly 22, one or moretorque bars 24, a wheel rotational axis 26, a wheel well recess 28, anactuator 30, multiple brake rotors 32, multiple brake stators 34, apressure plate 36, an end plate 38, a heat shield 40, multiple heatshield sections 42, multiple heat shield carriers 44, an air gap 46,multiple torque bar bolts 48, a torque bar pin 50, a wheel web hole 52,multiple heat shield fasteners 53, multiple rotor lugs 54, and multiplestator slots 56. FIG. 1B illustrates a portion of aircraft wheel brakingassembly 10 as viewed into wheel well 18 and wheel well recess 28.

In various embodiments, the various components of aircraft wheel brakingassembly 10 may be subjected to the application of compositions andmethods for protecting the components from oxidation.

Brake disks (e.g., interleaved rotors 32 and stators 34) are disposed inwheel well recess 28 of wheel well 18. Rotors 32 are secured to torquebars 24 for rotation with wheel 14, while stators 34 are engaged withtorque take-out assembly 22. At least one actuator 30 is operable tocompress interleaved rotors 32 and stators 34 for stopping the aircraft.In this example, actuator 30 is shown as a hydraulically actuatedpiston, but many types of actuators are suitable, such as anelectromechanical actuator. Pressure plate 36 and end plate 38 aredisposed at opposite ends of the interleaved rotors 32 and stators 34.Rotors 32 and stators 34 can comprise any material suitable for frictiondisks, including ceramics or carbon materials, such as a carbon/carboncomposite.

Through compression of interleaved rotors 32 and stators 34 betweenpressure plates 36 and end plate 38, the resulting frictional contactslows rotation of wheel 14. Torque take-out assembly 22 is secured to astationary portion of the landing gear truck such as a bogie beam orother landing gear strut, such that torque take-out assembly 22 andstators 34 are prevented from rotating during braking of the aircraft.

Carbon-carbon composites (also referred to herein as compositestructures, composite substrates, and carbon-carbon compositestructures, interchangeably) in the friction disks may operate as a heatsink to absorb large amounts of kinetic energy converted to heat duringslowing of the aircraft. Heat shield 40 may reflect thermal energy awayfrom wheel well 18 and back toward rotors 32 and stators 34. Withreference to FIG. 1A, a portion of wheel well 18 and torque bar 24 isremoved to better illustrate heat shield 40 and heat shield segments 42.With reference to FIG. 1B, heat shield 40 is attached to wheel 14 and isconcentric with wheel well 18. Individual heat shield sections 42 may besecured in place between wheel well 18 and rotors 32 by respective heatshield carriers 44 fixed to wheel well 18. Air gap 46 is definedannularly between heat shield segments 42 and wheel well 18.

Torque bars 24 and heat shield carriers 44 can be secured to wheel 14using bolts or other fasteners. Torque bar bolts 48 can extend through ahole formed in a flange or other mounting surface on wheel 14. Eachtorque bar 24 can optionally include at least one torque bar pin 50 atan end opposite torque bar bolts 48, such that torque bar pin 50 can bereceived through wheel web hole 52 in web 20. Heat shield sections 42and respective heat shield carriers 44 can then be fastened to wheelwell 18 by heat shield fasteners 53.

Under the operating conditions (e.g., high temperature) of aircraftwheel braking assembly 10, carbon-carbon composites may be prone tomaterial loss from oxidation of the carbon. For example, variouscarbon-carbon composite components of aircraft wheel braking assembly 10may experience both catalytic oxidation and inherent thermal oxidationcaused by heating the composite during operation. In variousembodiments, composite rotors 32 and stators 34 may be heated tosufficiently high temperatures that may oxidize the carbon surfacesexposed to air. At elevated temperatures, infiltration of air andcontaminants may cause internal oxidation and weakening, especially inand around brake rotor lugs 54 or stator slots 56 securing the frictiondisks to the respective torque bar 24 and torque take-out assembly 22.Because carbon-carbon composite components of aircraft wheel brakingassembly 10 may retain heat for a substantial time period after slowingthe aircraft, oxygen from the ambient atmosphere may react with thecarbon matrix and/or carbon fibers to accelerate material loss. Further,damage to brake components may be caused by the oxidation enlargement ofcracks around fibers or enlargement of cracks in a reaction-formedporous barrier coating (e.g., a silicon-based barrier coating) appliedto the carbon-carbon composite.

Elements identified in severely oxidized regions of carbon-carboncomposite brake components include potassium (K) and sodium (Na). Thesealkali contaminants may come into contact with aircraft brakes as partof cleaning or de-icing materials. Other sources include salt depositsleft from seawater or sea spray. These and other contaminants (e.g. Ca,Fe, etc.) can penetrate and leave deposits in pores of carbon-carboncomposite aircraft brakes, including the substrate and anyreaction-formed porous barrier coating. When such contamination occurs,the rate of carbon loss by oxidation can be increased by one to twoorders of magnitude.

In various embodiments, components of aircraft wheel braking assembly 10may reach operating temperatures in the range from about 100° C. (212°F.) up to about 900° C. (1652° F.). However, it will be recognized thatthe oxidation protection compositions and methods of the presentdisclosure may be readily adapted to many parts in this and otherbraking assemblies, as well as to other carbon-carbon compositestructures susceptible to oxidation losses from infiltration ofatmospheric oxygen and/or catalytic contaminants.

In various embodiments, a method for limiting an oxidation reaction in acomposite structure may comprise forming a first slurry by combining afirst pre-slurry composition comprising a first phosphate glasscomposition in the form of a glass frit, powder, or other suitablepulverized form, with a first carrier fluid (such as, for example,water), applying the first slurry to a composite structure, and heatingthe composite structure to a temperature sufficient to dry the carrierfluid and form an oxidation protection coating on the compositestructure, which in various embodiments may be referred to a base layer.The first pre-slurry composition of the first slurry may compriseadditives, such as, for example, ammonium hydroxide, ammonium dihydrogenphosphate, nanoplatelets (such as graphene-based nanoplatelets), amongothers, to improve hydrolytic stability and/or to increase the compositestructure's resistance to oxidation, thereby tending to reduce mass lossof composite structure. In various embodiments, a slurry comprising acidaluminum phosphates having an aluminum (Al) to phosphoric acid (H₃PO₄)ratio of 1 to 3 or less by weight, such as an Al:H₃PO₄ ratio of between1 to 2 and 1 to 3 by weight, tends to provide increased hydrolyticstability without substantially increasing composite structure massloss. In various embodiments, a slurry comprising acid aluminumphosphates having an Al:H₃PO₄ ratio between 1:2 to 1:3 produces anincrease in hydrolytic protection and an unexpected reduction incomposite structure mass loss.

With initial reference to FIG. 2A, a method 200 for coating a compositestructure in accordance with various embodiments is illustrated. Method200 may, for example, comprise applying an oxidation inhibitingcomposition to non-wearing surfaces of carbon-carbon composite brakecomponents. In various embodiments, method 200 may be used on the backface of pressure plate 36 and/or end plate 38, an inner diameter (ID)surface of stators 34 including slots 56, as well as outer diameter (OD)surfaces of rotors 32 including lugs 54. The oxidation inhibitingcomposition of method 200 may be applied to preselected regions of acarbon-carbon composite structure that may be otherwise susceptible tooxidation. For example, aircraft brake disks may have the oxidationinhibiting composition applied on or proximate stator slots 56 and/orrotor lugs 54.

In various embodiments, method 200 may comprise forming a first slurry(step 210) by combining a first pre-slurry composition, comprising afirst phosphate glass composition in the form of a glass frit, powder,or other suitable pulverized and/or ground form, with a first carrierfluid (such as, for example, water). In various embodiments, the firstslurry may comprise an acid aluminum phosphate wherein the ratio ofAl:H₃PO₄ may be between 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5to 1:3, between 1:2.7 to 1:3 or between 1:2.9 to 1:3, as measured byweight. The first pre-slurry composition of the first slurry may furthercomprise a boron nitride additive. For example, a boron nitride (such ashexagonal boron nitride) may be added to the first phosphate glasscomposition such that the resulting first pre-slurry compositioncomprises between about 10 weight percent and about 30 weight percent ofboron nitride, wherein the term “about” in this context only means plusor minus 2 weight percent. Further, the pre-slurry composition maycomprise between about 15 weight percent and 25 weight percent of boronnitride, wherein the term “about” in this context only means plus orminus 2 weight percent. Boron nitride may be prepared for addition tothe first phosphate glass composition by, for example, ultrasonicallyexfoliating boron nitride in dimethylformamide (DMF), a solution of DMFand water, or 2-propanol solution. In various embodiments, the boronnitride additive may comprise a boron nitride that has been prepared foraddition to the first phosphate glass composition by crushing or milling(e.g., ball milling) the boron nitride. The resulting boron nitride maybe combined with the first phosphate glass composition glass frit.

The first phosphate glass composition may comprise one or more alkalimetal glass modifiers, one or more glass network modifiers and/or one ormore additional glass formers. In various embodiments, boron oxide or aprecursor may optionally be combined with the P₂O₅ mixture to form aborophosphate glass, which has improved self-healing properties at theoperating temperatures typically seen in aircraft braking assemblies. Invarious embodiments, the phosphate glass and/or borophosphate glass maybe characterized by the absence of an oxide of silicon. Further, theratio of P₂O₅ to metal oxide in the fused glass may be in the range fromabout 0.25 to about 5 by weight.

Potential alkali metal glass modifiers may be selected from oxides oflithium, sodium, potassium, rubidium, cesium, and mixtures thereof. Invarious embodiments, the glass modifier may be an oxide of lithium,sodium, potassium, or mixtures thereof. These or other glass modifiersmay function as fluxing agents. Additional glass formers can includeoxides of boron, silicon, sulfur, germanium, arsenic, antimony, andmixtures thereof.

Suitable glass network modifiers include oxides of vanadium, aluminum,tin, titanium, chromium, manganese, iron, cobalt, nickel, copper,mercury, zinc, thulium, lead, zirconium, lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium,uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin,bismuth, cadmium, and mixtures thereof.

The first phosphate glass composition may be prepared by combining theabove ingredients and heating them to a fusion temperature. In variousembodiments, depending on the particular combination of elements, thefusion temperature may be in the range from about 700° C. (1292° F.) toabout 1500° C. (2732° F.). The resultant melt may then be cooled andpulverized and/or ground to form a glass frit or powder. In variousembodiments, the first phosphate glass composition may be annealed to arigid, friable state prior to being pulverized. Glass transitiontemperature (T_(g)), glass softening temperature (T_(s)) and glassmelting temperature (T_(m)) may be increased by increasing refinementtime and/or temperature. Before fusion, the first phosphate glasscomposition comprises from about 20 mol % to about 80 mol % of P₂O₅. Invarious embodiments, the first phosphate glass composition comprisesfrom about 30 mol % to about 70 mol % P₂O₅, or precursor thereof. Invarious embodiments, the first phosphate glass composition comprisesfrom about 40 to about 60 mol % of P₂O₅.

The first phosphate glass composition may comprise from about 5 mol % toabout 50 mol % of the alkali metal oxide. In various embodiments, thefirst phosphate glass composition comprises from about 10 mol % to about40 mol % of the alkali metal oxide. Further, the first phosphate glasscomposition comprises from about 15 to about 30 mol % of the alkalimetal oxide or one or more precursors thereof. In various embodiments,the first phosphate glass composition may comprise from about 0.5 mol %to about 50 mol % of one or more of the above-indicated glass formers.The first phosphate glass composition may comprise about 5 to about 20mol % of one or more of the above-indicated glass formers. As usedherein, mol % is defined as the number of moles of a constituent per thetotal moles of the solution.

In various embodiments, the first phosphate glass composition cancomprise from about 0.5 mol % to about 40 mol % of one or more of theabove-indicated glass network modifiers. The first phosphate glasscomposition may comprise from about 2.0 mol % to about 25 mol % of oneor more of the above-indicated glass network modifiers.

In various embodiments, the first phosphate glass composition, excludingthe low CTE material, may represented by the formula:

a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z)  [1]

In Formula 1, A′ is selected from: lithium, sodium, potassium, rubidium,cesium, and mixtures thereof; G_(f) is selected from: boron, silicon,sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ isselected from: vanadium, aluminum, tin, titanium, chromium, manganese,iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium,lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,actinium, thorium, uranium, yttrium, gallium, magnesium, calcium,strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is anumber in the range from 1 to about 5; b is a number in the range from 0to about 10; c is a number in the range from 0 to about 30; x is anumber in the range from about 0.050 to about 0.500; y₁ is a number inthe range from about 0.100 to about 0.950; y₂ is a number in the rangefrom 0 to about 0.20; and z is a number in the range from about 0.01 toabout 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂). The first phosphate glasscomposition may be formulated to balance the reactivity, durability andflow of the resulting glass barrier layer for optimal performance.

In various embodiments, first phosphate glass composition in glass fritform may be combined with additional components to form the firstpre-slurry composition. For example, crushed first phosphate glasscomposition in glass frit form may be combined with ammonium hydroxide,ammonium dihydrogen phosphate, nanoplatelets (such as graphene-basednanoplatelets), among other materials and/or substances. For example,graphene nanoplatelets could be added to the first phosphate glasscomposition in glass frit form. In various embodiments, the additionalcomponents may be combined and preprocessed before combining them withfirst phosphate glass composition in glass frit form. Other suitableadditional components include, for example, surfactants such as, forexample, an ethoxylated low-foam wetting agent and flow modifiers, suchas, for example, polyvinyl alcohol, polyacrylate, or similar polymers.In various embodiments, other suitable additional components may includeadditives to enhance impact resistance and/or to toughen the barriercoating, such as, for example, at least one of whiskers, nanofibers ornanotubes consisting of nitrides, carbides, carbon, graphite, quartz,silicates, aluminosilicates, phosphates, and the like. In variousembodiments, additives to enhance impact resistance and/or to toughenthe barrier coating may include silicon carbide whiskers, carbonnanofibers, boron nitride nanotubes and similar materials known to thoseskilled in the art.

The components in the first pre-slurry composition may have coefficientsof thermal expansion (“CTE”) that are significantly higher than thecoefficient of thermal expansion of carbon composite material in acomposite structure, to which the first slurry may be applied. The CTEof carbon composite materials may range from below zero to about3.6×10⁻⁶° C.⁻¹. The CTE of glass phosphate, on the other hand, is higherthan that of carbon composite materials, ranging from about 12×10⁻⁶°C.⁻¹¹ to about 14×10⁻⁶° C.⁻¹, wherein the term “about” as used in thiscontext only refers to plus or minus 2×10⁻⁶° C.⁻¹. Therefore, duringoperation, the thermal expansion of glass phosphate in the firstpre-slurry composition of the first slurry may cause cracks in thecarbon material in the composite structure, because the carbon materialexpands at a slower rate than the phosphate glass, and other materialsin the first slurry, when exposed to heat.

In various embodiments, materials with low CTEs may be added to thefirst pre-slurry composition. Materials with low CTEs may be materialshaving CTEs of 10×10⁻⁶° C.⁻¹ or less, or further, materials having CTEsof 3×10⁻⁶° C.⁻¹ or less. For example, a low CTE material added to thefirst pre-slurry composition may be beta-spodumene (Li₂O*Al₂0₃*2SiO₂)with a CTE of 0.9×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.),beta-eucryptite (Li₂O*Al₂O₃*2SiO₂) with a CTE of −6.2×10⁻⁶° C.⁻¹ at 25°C. (77° F.) to 1000° C. (1832° F.), cordierite (Mg₂A₁₄Si₅O₁₈) with a CTEof 1.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 800° C. (1472° F.), faujasite((Na₂,Ca,Mg)_(3.5)[Al₇Si₁₇O₄₈].32(H₂O)) with a CTE of −3.24×10⁻⁶° C.⁻¹,NZP materials such as SrZr₄P₆O₂₄ and CaZr₄P₆O₂₄ with CTEs of 0.6×10⁻⁶°C. at 25° C. (77° F.) to 1000° C. (1832° F.) and NaZr₂P₃O₁₂ with a CTEof −0.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), Al₂Mo₃O₁₂with a CTE of 2.4×10⁻⁶° C.⁻¹, Al₂W₃O₁₂ with a CTE of 2.2×10⁻⁶° C.⁻¹,MoZr₂P₂O₁₂, WZr₂P₂O₁₂, niobium pentoxide (Nb₂O₅) with a CTE of 1.0×10⁻⁶°C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), aluminum titanate(Al₂TiO₅) with a CTE of 1.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 800° C.(1472° F.), Zr₂P₂O₉ with a CTE of 0.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to600° C. (1112° F.), Beryl (Be₃Al₂Si₆O₁₈) with a CTE of 2.0×10⁻⁶° C.⁻¹ at25° C. (77° F.) to 1000° C. (1832° F.), silicon dioxide glass (SiO₂)with a CTE of 0.5×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.),SiO₂—TiO₂ glass with a CTE of −0.03×10⁻⁶° C.⁻¹ to 0.05×10⁻⁶° C.⁻¹ at 25°C. (77° F.) to 800° C. (1472° F.), Cu₂O—Al₂O₃—SiO₂ glasses with a CTE of0.5×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 500° C. (932° F.), ZERODUR (amaterial comprising, by weight, 57.2% SiO₂, 25.3% Al₂O₃, 6.5% P₂O₅, 3.4%Li₂O, 2.3% TiO₂, 1.8% ZrO₂, 1.4% ZnO, 1.0% MgO, 0.5% As₂O₃, 0.4% K₂O,and 0.2% Na₂O) with a CTE of 0.12×10⁻⁶° C.⁻¹ at 20° C. (68° F.) to 600°C. (1112° F.), and/or lead magnesium niobate (Pb₃MgNb₂O₉) with a CTE of1.0×10⁻⁶° C.⁻¹ at −100° C. (−148° F.) to 100° C. (212° F.). In variousembodiments, the first slurry may comprise an amount between 0.5% byweight and 50% by weight of the low CTE material. In variousembodiments, the first slurry may comprise an amount between 0.5% byweight and 40% by weight of the low CTE material. In variousembodiments, the first slurry may comprise an amount between 5% byweight and 30% by weight of the low CTE material. In variousembodiments, the first pre-slurry composition may comprise between 0.5%by weight and 95% by weight of the low CTE material, wherein the firstpre-slurry composition comprises all the components of the first slurryexcept the first carrier fluid. In various embodiments, the firstpre-slurry composition may comprise between 1% by weight and 90% byweight of the low CTE material. In various embodiments, the firstpre-slurry composition may comprise between 5% by weight and 80% byweight of the low CTE material. In various embodiments, the firstpre-slurry composition may comprise between 10% by weight and 30% byweight of the low CTE material.

By adding a low CTE material, such as those listed herein, to the firstpre-slurry composition, the difference between the CTE of the carbonmaterial in the composite structure and the average CTE of the firstslurry, which will form the base layer after heating (step 230,described below), may be decreased. Therefore, by tailoring the thermalexpansion properties of the base layer through the addition of one ormore low CTE materials, in response to being exposed to heat, the baselayer and the carbon material in the composite structure may expand atmore similar rates than a base layer without a low CTE material. Moresimilar CTEs between the carbon material in the composite structure andthe base layer formed by the first slurry may help maintain thestructural integrity of the composite structure by alleviating crackingin the carbon material and/or cracking in the base layer in response tobeing heated.

In various embodiments, method 200 further comprises applying the firstslurry to a composite structure (step 220). Applying the first slurrymay comprise, for example, spraying or brushing the first slurry of thefirst phosphate glass composition on to an outer surface of thecomposite structure. Any suitable manner of applying the base layer tothe composite structure is within the scope of the present disclosure.As referenced herein, the composite structure may refer to acarbon-carbon composite structure.

In various embodiments, method 200 further comprises a step 230 ofheating the composite structure to form a base layer of phosphate glass.The composite structure may be heated (e.g., dried or baked) at atemperature in the range from about 200° C. (292° F.) to about 1000° C.(1832° F.). In various embodiments, the composite structure is heated toa temperature in a range from about 600° C. (1112° F.) to about 1000° C.(1832° F.), or between about 200° C. (292° F.) to about 900° C. (1652°F.), or further, between about 400° C. (752° F.) to about 850° C. (1562°F.). Step 230 may, for example, comprise heating the composite structurefor a period between about 0.5 hour and about 8 hours, wherein the term“about” in this context only means plus or minus 0.25 hours. The baselayer may also be referred to as a coating.

In various embodiments, the composite structure may be heated to afirst, lower temperature (for example, about 30° C. (86° F.) to about400° C. (752° F.)) to bake or dry the base layer at a controlled depth.A second, higher temperature (for example, about 300° C. (572° F.) toabout 1000° C. (1832° F.)) may then be used to form a deposit from thebase layer within the pores of the composite structure. The duration ofeach heating step can be determined as a fraction of the overall heatingtime and can range from about 10% to about 50%, wherein the term “about”in this context only means plus or minus 5%. In various embodiments, theduration of the lower temperature heating step(s) can range from about20% to about 40% of the overall heating time, wherein the term “about”in this context only means plus or minus 5%. The lower temperaturestep(s) may occupy a larger fraction of the overall heating time, forexample, to provide relatively slow heating up to and through the firstlower temperature. The exact heating profile will depend on acombination of the first temperature and desired depth of the dryingportion.

Step 230 may be performed in an inert environment, such as under ablanket of inert gas or less reactive gas (e.g., nitrogen, argon, othernoble gases and the like). For example, a composite structure may bepretreated or warmed prior to application of the base layer to aid inthe penetration of the base layer. Step 230 may be for a period of about2 hours at a temperature of about 600° C. (1112° F.) to about 800° C.(1472° F.), wherein the term “about” in this context only means plus orminus 10° C. The composite structure and base layer may then be dried orbaked in a non-oxidizing, inert or less reactive atmosphere, e.g., noblegasses and/or nitrogen (N₂), to optimize the retention of the firstpre-slurry composition of the base layer in the pores of the compositestructure. This retention may, for example, be improved by heating thecomposite structure to about 200° C. (392° F.) and maintaining thetemperature for about 1 hour before heating the carbon-carbon compositeto a temperature in the range described above. The temperature rise maybe controlled at a rate that removes water without boiling, and providestemperature uniformity throughout the composite structure.

In various embodiments and with reference now to FIG. 2B, method 300,which comprises steps also found in method 200, may further compriseapplying at least one of a pretreating composition or a barrier coating(step 215) prior to applying the first slurry. Step 215 may, forexample, comprise applying a first pretreating composition to an outersurface of a composite structure, such as a component of aircraft wheelbraking assembly 10. In various embodiments, the first pretreatingcomposition comprises an aluminum oxide in water. For example, thealuminum oxide may comprise an additive, such as a nanoparticledispersion of aluminum oxide (for example, NanoBYK-3600®, sold by BYKAdditives & Instruments). The first pretreating composition may furthercomprise a surfactant or a wetting agent. The composite structure may beporous, allowing the pretreating composition to penetrate at least aportion of the pores of the composite structure.

In various embodiments, after applying the first pretreatingcomposition, the component may be heated to remove water and fix thealuminum oxide in place. For example, the component may be heatedbetween about 100° C. (212° F.) and 200° C. (392° F.), and further,between 100° C. (212° F.) and 150° C. (302° F.).

Step 215 may further comprise applying a second pretreating composition.In various embodiments, the second pretreating composition comprises aphosphoric acid and an aluminum phosphate, aluminum hydroxide, and/oraluminum oxide. The second pretreating composition may further comprise,for example, a second metal salt such as a magnesium salt. In variousembodiments, the aluminum to phosphorus ratio of the aluminum phosphateis 1 to 3 or less by weight. Further, the second pretreating compositionmay also comprise a surfactant or a wetting agent. In variousembodiments, the second pretreating composition is applied to thecomposite structure atop the first pretreating composition. Thecomposite structure may then, for example, be heated. In variousembodiments, the composite structure may be heated between about 600° C.(1112° F.) and about 800° C. (1472° F.), and further, between about 650°C. (1202° F.) and 750° C. (1382° F.).

Step 215 may further comprise applying a barrier coating to an outersurface of a composite structure, such as a component of aircraft wheelbraking assembly 10. In various embodiments the barrier coatingcomposition may comprise carbides or nitrides, including at least one ofa boron nitride, silicon carbide, titanium carbide, boron carbide,silicon oxycarbide, and silicon nitride. In various embodiments, thebarrier coating may be formed by treating the composite structure withmolten silicon. The molten silicon is reactive and may form a siliconcarbide barrier on the composite structure. Step 215 may comprise, forexample, application of the barrier coating by spraying, chemical vapordeposition (CVD), molten application, or brushing the barrier coatingcomposition on to the outer surface of the carbon-carbon compositestructure. Any suitable manner of applying the base layer to compositestructure is within the scope of the present disclosure.

In various embodiments and with reference now to FIG. 2C, method 400 mayfurther comprise a step 240, similar to step 210, of forming a secondslurry by combining a second pre-slurry composition, which may comprisea second phosphate glass composition in glass frit or powder form, witha second carrier fluid (such as, for example, water). In variousembodiments, the second slurry may comprise an acid aluminum phosphatewherein the ratio of aluminum (Al) to phosphoric acid (H₃PO₄) may bebetween 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5 to 1:3, between1:2.7 to 1:3 or between 1:2.9 to 1:3. In various embodiments, the secondslurry may comprise a second pre-slurry composition comprising acidaluminum phosphate and orthophosphoric acid with an aluminum tophosphate ratio of 1:2 to 1:5, and may be substantially phosphate glassfree. As used herein “substantially free” means comprising less than0.01% by weight of a substance. Further, step 240 may comprise sprayingor brushing the second slurry of the second phosphate glass compositionon to an outer surface of the base layer. Any suitable manner ofapplying the sealing layer to the base layer is within the scope of thepresent disclosure.

In various embodiments, the second slurry may be substantially free ofboron nitride. In this case, “substantially free” means less than 0.01percent by weight. For example, the second pre-slurry composition maycomprise any of the components of the pre-slurry compositions and/orglass compositions described in connection with the first pre-slurrycomposition and/or first phosphate glass composition, without theaddition of a boron nitride additive. In various embodiments, the secondpre-slurry mixture may comprise the same pre-slurry composition and/orphosphate glass composition used to prepare the first pre-slurrycomposition and/or the first phosphate glass composition. In variousembodiments, the second pre-slurry composition may comprise a differentpre-slurry composition and/or phosphate glass composition than the firstpre-slurry composition and/or first phosphate glass composition.

In various embodiments, the first slurry and/or the second slurry maycomprise an additional metal salt. The cation of the additional metalsalt may be multivalent. The metal may be an alkaline earth metal or atransition metal. In various embodiments, the metal may be an alkalimetal. The multivalent cation may be derived from a non-metallic elementsuch as boron. The term “metal” is used herein to include multivalentelements such as boron that are technically non-metallic. The metal ofthe additional metal salt may be an alkaline earth metal such ascalcium, magnesium, strontium, barium, or a mixture of two or morethereof. The metal for the additional metal salt may be iron, manganese,tin, zinc, or a mixture of two or more thereof. The anion for theadditional metal salt may be an inorganic anion such as a phosphate,halide, sulfate or nitrate, or an organic anion such as acetate. In oneembodiment, the additional metal salt may be an alkaline earth metalsalt such as an alkaline earth metal phosphate. In one embodiment, theadditional metal salt may be a magnesium salt such as magnesiumphosphate. In one embodiment, the additional metal salt may be analkaline earth metal nitrate, an alkaline earth metal halide, analkaline earth metal sulfate, an alkaline earth metal acetate, or amixture of two or more thereof. In one embodiment, the additional metalsalt may be magnesium nitrate, magnesium halide, magnesium sulfate, or amixture of two or more thereof. In one embodiment, the additional metalsalt may comprise: (i) magnesium phosphate; and (ii) a magnesiumnitrate, magnesium halide, magnesium sulfate, or a mixture of two ormore thereof.

The additional metal salt may be selected with reference to itscompatibility with other ingredients in the first slurry and/or thesecond slurry. Compatibility may include metal phosphates that do notprecipitate, flocculate, agglomerate, react to form undesirable species,or settle out prior to application of the first slurry and/or the secondslurry to the carbon-carbon composite. The phosphates may be monobasic(H₂PO₄ ⁻), dibasic (HPO₄ ⁻²), or tribasic (PO₄ ⁻³). The phosphates maybe hydrated. Examples of alkaline earth metal phosphates that may beused include calcium hydrogen phosphate (calcium phosphate, dibasic),calcium phosphate tribasic octahydrate, magnesium hydrogen phosphate(magnesium phosphate, dibasic), magnesium phosphate tribasicoctahydrate, strontium hydrogen phosphate (strontium phosphate,dibasic), strontium phosphate tribasic octahydrate and barium phosphate.

In one embodiment, a chemical equivalent of the additional metal saltmay be used as the additional metal salt. Chemical equivalents includecompounds that yield an equivalent (in this instance, an equivalent ofthe additional metal salt) in response to an outside stimulus such as,temperature, hydration, or dehydration. For example, equivalents ofalkaline earth metal phosphates may include alkaline earth metalpyrophosphates, hypophosphates, hypophosphites and orthophosphites.Equivalent compounds include magnesium and barium pyrophosphate,magnesium and barium orthophosphate, magnesium and barium hypophosphate,magnesium and barium hypophosphite, and magnesium and bariumorthophosphite.

While not wishing to be bound by theory, it is believed that theaddition of multivalent cations, such as alkaline earth metals,transition metals and nonmetallic elements such as boron, to the firstslurry and/or the second slurry enhances the hydrolytic stability of themetal-phosphate network. In general, the hydrolytic stability of themetal-phosphate network increases as the metal content increases,however a change from one metallic element to another may influenceoxidation inhibition to a greater extent than a variation in themetal-phosphate ratio. The solubility of the phosphate compounds may beinfluenced by the nature of the cation associated with the phosphateanion. For example, phosphates incorporating monovalent cations such assodium orthophosphate or phosphoric acid (hydrogen cations) are verysoluble in water while (tri)barium orthophosphate is insoluble.Phosphoric acids can be condensed to form networks but such compoundstend to remain hydrolytically unstable. Generally, it is believed thatthe multivalent cations link phosphate anions creating a phosphatenetwork with reduced solubility. Another factor that may influencehydrolytic stability is the presence of —P—O—H groups in the condensedphosphate product formed from the first slurry and/or the second slurryduring thermal treatment. The first slurry and/or the second slurry maybe formulated to minimize concentration of these species and anysubsequent hydrolytic instability. Whereas increasing the metal contentmay enhance the hydrolytic stability of the first slurry and/or thesecond slurry, it may be desirable to strike a balance betweencomposition stability and effectiveness as an oxidation inhibitor.

In various embodiments, the additional metal salt may be present in thefirst slurry and/or the second slurry at a concentration in the rangefrom about 0.5 weight percent to about 30 weight percent, and in variousembodiments from about 0.5 weight percent to about 25 weight percent,and in various embodiments from about 5 weight percent to about 20weight percent. In various embodiments, a combination of two or moreadditional metal salts may be present at a concentration in the rangefrom about 10 weight percent to about 30 weight percent, and in variousembodiments from about 12 weight percent to about 20 weight percent.

Method 400 may further comprise a step 250 of heating the compositestructure to form a sealing layer, which may comprise phosphate glass,over the base layer. Similar to step 230, the composite structure may beheated at a temperature sufficient to adhere the sealing layer to thebase layer by, for example, drying or baking the carbon-carbon compositestructure at a temperature in the range from about 200° C. (392° F.) toabout 1000° C. (1832° F.). In various embodiments, the compositestructure is heated to a temperature in a range from about 600° C.(1112° F.) to about 1000° C. (1832° F.), or between about 200° C. (392°F.) to about 900° C. (1652° F.), or further, between about 400° C. (752°F.) to about 850° C. (1562° F.), wherein in this context only, the term“about” means plus or minus 10° C. Further, step 250 may, for example,comprise heating the composite structure for a period between about 0.5hour and about 8 hours, where the term “about” in this context onlymeans plus or minus 0.25 hours.

In various embodiments, step 250 may comprise heating the compositestructure to a first, lower temperature (for example, about 30° C. (86°F.) to about 300° C. (572° F.)) followed by heating at a second, highertemperature (for example, about 300° C. (572° F.) to about 1000° C.(1832° F.)). Further, step 250 may be performed in an inert environment,such as under a blanket of inert or less reactive gas (e.g., nitrogen,argon, other noble gases, and the like).

TABLE 1 illustrates a variety of slurries comprising pre-slurrycompositions, including phosphate glass compositions, prepared inaccordance with various embodiments.

TABLE 1 Example A B C D E F G H h-Boron nitride powder 0 0 7.50 6.258.125 0 0 1.0 Graphene nanoplatelets 0 0.15 0.15 0.15 0.15 0.15 0.150.15 β-Spodumene 0 8.75 1.25 2.50 0.675 2.50 2.50 35.0 H₂O 52.40 50.0050.00 50.00 50.00 50.00 50.00 50.00 Surfynol 465 surfactant 0 0.20 0.200.20 0.20 0.20 0.20 0.20 Ammonium dihydrogen phosphate (ADHP) 11.33 0.500.50 0.50 0.50 0.50 0.50 0.50 Glass frit 34.00 26.50 26.50 26.50 26.5026.50 26.50 26.50 Aluminum orthophosphate (o-AlPO₄) 2.270 2.270 0 0 0 02.270 0

As illustrated in TABLE 1, oxidation protection system slurriescomprising a pre-slurry composition, comprising phosphate glasscomposition glass frit and various additives including h-boron nitride,graphene nanoplatelets, a surfactant, a flow modifier such as, forexample, polyvinyl alcohol, polyacrylate or similar polymer, ammoniumdihydrogen phosphate, and/or ammonium hydroxide, in a carrier fluid(i.e., water) were prepared. Slurry A may be a suitable second slurrywhich will serve as a sealing layer when heated (such as during step250). Slurries B through H may comprise slurries comprising componentsdiscussed herein, including a low CTE material: beta-spodumene. Forexample, slurries B through H may illustrate suitable first slurrieswhich will form suitable base layers after heating (such as during step230), such as first slurries applied in step 220 of methods 200, 300,and 400.

Benefits and other advantages have been described herein with regard tospecific embodiments. Furthermore, the connecting lines shown in thevarious figures contained herein are intended to represent exemplaryfunctional relationships and/or physical couplings between the variouselements. It should be noted that many alternative or additionalfunctional relationships or physical connections may be present in apractical system. However, the benefits, advantages, solutions toproblems, and any elements that may cause any benefit, advantage, orsolution to occur or become more pronounced are not to be construed ascritical, required, or essential features or elements of the disclosure.The scope of the disclosure is accordingly to be limited by nothingother than the appended claims, in which reference to an element in thesingular is not intended to mean “one and only one” unless explicitly sostated, but rather “one or more.” Moreover, where a phrase similar to“at least one of A, B, or C” is used in the claims, it is intended thatthe phrase be interpreted to mean that A alone may be present in anembodiment, B alone may be present in an embodiment, C alone may bepresent in an embodiment, or that any combination of the elements A, Band C may be present in a single embodiment; for example, A and B, A andC, B and C, or A and B and C.

Systems, methods and apparatus are provided herein. In the detaileddescription herein, references to “one embodiment,” “an embodiment,” “anexample embodiment,” etc., indicate that the embodiment described mayinclude a particular feature, structure, or characteristic, but everyembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed. After reading the description, it will be apparent to oneskilled in the relevant art(s) how to implement the disclosure inalternative embodiments.

Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element herein is to be construed under theprovisions of 35 U.S.C. 112(f), unless the element is expressly recitedusing the phrase “means for.” As used herein, the terms “comprises,”“comprising,” or any other variation thereof, are intended to cover anon-exclusive inclusion, such that a process, method, article, orapparatus that comprises a list of elements does not include only thoseelements but may include other elements not expressly listed or inherentto such process, method, article, or apparatus.

What is claimed is:
 1. A method for coating a composite structure,comprising: forming a first slurry by combining a first pre-slurrycomposition with a first carrier fluid, wherein the first pre-slurrycomposition comprises a first phosphate glass composition and a lowcoefficient of thermal expansion material, wherein the low coefficientof thermal expansion material is a material comprising beta-eucryptitewith a coefficient of thermal expansion of less than 10×10⁻⁶° C.⁻¹;applying the first slurry on a surface of the composite structure; andheating the composite structure to a temperature sufficient to form abase layer on the composite structure.
 2. The method of claim 1, furthercomprising forming a second slurry by combining a second pre-slurrycomposition with a second carrier fluid, wherein the second pre-slurrycomposition comprises a second phosphate glass composition; applying thesecond slurry to the base layer; and heating the composite structure toa second temperature sufficient to form a sealing layer on the baselayer.
 3. The method of claim 1, further comprising applying at leastone of a pretreating composition or a barrier coating to the compositestructure prior to applying the first slurry to the composite structure.4. The method of claim 1, further comprising applying a pretreatingcomposition, wherein the applying comprises: applying a firstpretreating composition to an outer surface of the composite structure,the first pretreating composition comprising aluminum oxide and water;heating the pretreating composition; and applying a second pretreatingcomposition comprising at least one of a phosphoric acid or an acidphosphate salt, and an aluminum salt on the first pretreatingcomposition, wherein the composite structure is porous and the secondpretreating composition penetrates at least a pore of the compositestructure.
 5. The method of claim 3, wherein the barrier coatingcomprises at least one of a carbide, a nitride, a boron nitride, asilicon carbide, a titanium carbide, a boron carbide, a siliconoxycarbide, a molybdenum disulfide, a tungsten disulfide, or a siliconnitride.
 6. The method of claim 1, further comprising applying a barriercoating by at least one of reacting the composite structure with moltensilicon, spraying, chemical vapor deposition (CVD), molten application,or brushing.
 7. The method of claim 1, wherein the first pre-slurrycomposition of the base layer comprises between about 15 weight percentand about 30 weight percent of boron nitride.
 8. The method of claim 2,wherein at least one of the first phosphate glass composition or thesecond phosphate glass composition is represented by the formulaa(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z): A′ is selected from:lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum,tin, titanium, chromium, manganese, iron, cobalt, nickel, copper,mercury, zinc, thulium, lead, zirconium, lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium,uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin,bismuth, cadmium, and mixtures thereof; a is a number in the range from1 to about 5; b is a number in the range from 0 to about 10; c is anumber in the range from 0 to about 30; x is a number in the range fromabout 0.050 to about 0.500; y₁ is a number in the range from about 0.100to about 0.950; y₂ is a number in the range from 0 to about 0.20; and zis a number in the range from about 0.01 to about 0.5; (x+y₁+y₂+z)=1;and x<(y₁+y₂).
 9. The method of claim 1, wherein the first slurrycomprises a refractory compound such as a nitride, a boron nitride, asilicon carbide, a titanium carbide, a boron carbide, a siliconoxycarbide, silicon nitride, molybdenum disulfide, or tungstendisulfide.
 10. The method of claim 2, wherein at least one of the firstslurry or the second slurry comprises at least one of a surfactant, aflow modifier, a polymer, ammonium hydroxide, ammonium dihydrogenphosphate, acid aluminum phosphate, nanoplatelets, or graphenenanoplatelets.
 11. A slurry for coating a composite structure,comprising: a carrier fluid; and a first pre-slurry compositioncomprising a first phosphate glass composition and a low coefficient ofthermal expansion material, wherein the low coefficient of thermalexpansion material is a material comprising beta-eucryptite with acoefficient of thermal expansion of less than 10×10⁻⁶° C.⁻¹.
 12. Theslurry of claim 11, wherein the first pre-slurry composition comprisesbetween about 15 weight percent and 30 weight percent boron nitride. 13.The slurry of claim 11, wherein the first phosphate glass composition isrepresented by the formulaa(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z): A′ is selected from:lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum,tin, titanium, chromium, manganese, iron, cobalt, nickel, copper,mercury, zinc, thulium, lead, zirconium, lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium,uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin,bismuth, cadmium, and mixtures thereof; a is a number in the range from1 to about 5; b is a number in the range from 0 to about 10; c is anumber in the range from 0 to about 30; x is a number in the range fromabout 0.050 to about 0.500; y₁ is a number in the range from about 0.100to about 0.950; y₂ is a number in the range from 0 to about 0.20; and zis a number in the range from about 0.01 to about 0.5; (x+y₁+y₂+z)=1;and x<(y₁+y₂).
 14. The slurry of claim 11, further comprising arefractory compound such as a nitride, a boron nitride, a siliconcarbide, a titanium carbide, a boron carbide, a silicon oxycarbide,silicon nitride, molybdenum disulfide, or tungsten disulfide.
 15. Theslurry of claim 11, further comprising at least one of a surfactant, aflow modifier, a polymer, ammonium hydroxide, ammonium dihydrogenphosphate, acid aluminum phosphate, nanoplatelets, or graphenenanoplatelets.
 16. An article comprising: a carbon-carbon compositestructure; and an oxidation protection composition including a baselayer disposed on an outer surface of the carbon-carbon compositestructure, wherein the base layer comprises a first pre-slurrycomposition having a low coefficient of thermal expansion material,wherein the low coefficient of thermal expansion material is a materialcomprising beta-eucryptite with a coefficient of thermal expansion ofless than 10×10⁻⁶° C.⁻¹.
 17. The article of claim 16, further comprisinga sealing layer disposed on an outer surface of the base layer, whereinthe sealing layer comprises a second pre-slurry composition comprising asecond phosphate glass composition and acid aluminum phosphate.